NARRATOR: Ah, Venice. To Italians she's known as La
Serenissima, the Most Serene Republic. For twelve hundred years, the city has
performed a magical balancing act.

Not quite land, not quite sea, Venice seems to float in its own world. A
world that at first glance appears immune to the passing of time. The
nine-hundred-year-old St. Mark's Basilica presides over a giant public square
that's changed little since the days of Casanova and Vivaldi. But in recent
years, the illusion that is Venice has begun to crack, and this idyllic city
has been showing a darker side.

The trouble began on November 4, 1966, when an extremely high tide swept
into Venice and refused to leave. For 15 hours, Venice was inundated by the
sea. In historic Saint Mark's Square the water was four feet deep. Luckily, no
one was killed. But the place was a disaster zone.

In a single day, the city and the world were forced to face a harsh
reality: Venice was sinking into the sea.

Today flooding has become a fact of life. Instead of floating above the
water, the 15th and 16th century buildings are often
filled with it, and the ancient bricks are gradually dissolving away.

CHIANG C. MEI (Massachusetts Institute of Technology): Venice is
at a critical point. The problem of flooding must be solved right
away.

NARRATOR: But how can Venetians stop the flooding? Not only is
the city sinking, but sea level is rising here and all over the world.

EDMUND PENNING-ROWSELL (Flood Hazard Research Center): It's
happening. It's happening now. Venice is a trigger. Venice
is the first major city in the world to face sea level rise, because it's built
right at sea level.

CHARLES E. MCCLENNEN (Colgate University): The Venice you know
today cannot be preserved as it is today. It was built on a salt marsh
at sea level, in a sinking area, and unfortunately, sea level is rising.

NARRATOR: Is Venice's long and happy marriage to the sea
destined for disaster? Or can the City of Canals somehow be saved?

Sinking City of Venice up next on NOVA.

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NARRATOR: Several times a year, sirens sound along the canals
of Venice. They can mean only one thing: the water is coming.

Between October and January, the lowest parts of Venice flood almost every
day, and much of the city is inundated half a dozen times. During this wet
season, Venetians rarely venture far without their rubber boots. And
sometimes even that's not enough. They call it "acqua alta," high water.

ALBERT AMMERMAN (Colgate University): It's this sort of slimy,
murky water, which isn't very pleasant. But it also contains quite a bit
of pollution. So you really don't want to stick your hand in this water
or have this water in your entry to your house, in your hallway. Schools
can't run. Boats have troubles getting under the bridges. There's
a whole series of dysfunctional things which happen with acqua alta.

NARRATOR: It's not just inconvenient. The tide is insidious,
creeping into every building without regard for its historical value. Salt
water eats away at floors and walls, no matter what century they were built in
or what genius architect designed them. The front of St. Mark's Basilica,
perhaps the most famous Venetian landmark, is adorned with stones from around
the Mediterranean. All of it is being corroded by floods, almost on a daily
basis.

Venice is famous as the city of romance, the city of Casanova, where lovers
forget their worldly woes. It was once an extremely rich and powerful city,
built by merchants and bankers who controlled a shipping empire throughout the
Mediterranean. Her wealth and beauty flowed directly from the sea.

So how is it that the water is now threatening to undo all that it made? In
the case of Venice, it comes down to "location, location, location." If you
were planning to build a city, you could hardly pick a less practical spot.

At the northern tip of the Adriatic Sea, historic Venice sits on what is
actually several dozen islands, within a 200-square-mile shallow lagoon. A long
chain of barrier islands guards the lagoon from the sea, but three openings in
the chain allow ships, and the tide, access to the city. Flooding occurs when
exceptionally high tides break through these inlets.

Normally, the tides are controlled by the moon, but the sun also plays a
role. These astronomical tides are easily predicted, and they aren't very
extreme. The difference between high and low tide in Venice can be as little as
an inch or two, or it can be more than three feet, during a full or new moon.
But you can't blame the moon for Venice's flooding problems.

CHARLES E. MCCLENNEN: The acqua alta, those extreme high tides, are
caused primarily by the weather, strong winds out of the south driving Adriatic
waters to the north and forcing it into the lagoon and causing flooding.
So predicting acqua alta is really based on predicting storms. And
it's not that easy.

NARRATOR: Bad weather was the cause of the 1966 deluge. Wind
and low pressure created a giant storm surge that pushed water into the lagoon
and wouldn't let it out. The flood was a complete surprise.

Since then, Venice has paid much more attention to the weather and its
effect on the tides. Out on the Adriatic, twelve miles from shore, this
platform collects data 24 hours a day. Temperature and humidity, wind speed,
atmospheric pressure, anything that contributes to a storm surge. All this
information is relayed to a team within the city. If flooding is
predicted then alarms go off.

Given enough warning, Venetians can minimize the damage. Doorway barriers
keep some water out of ground floor shops and apartments, but for most of the
city, there's no stopping the sea.

These events are becoming more frequent. A century ago, St. Mark's Square,
the lowest point in Venice, flooded about nine times a year. Nowadays,
it happens about 100 times. Buildings in Venice were constructed to withstand
some contact with the sea, but these floods are pushing them beyond their
limits.

To understand why, just look at a typical Venetian foundation. Here, a
canal has been pumped dry for repairs, revealing a few of the secrets of the
city's success and some of its major weaknesses. Strip off the outer layers of
the foundation and you'll find a forest of wooden pylons. These logs were
pounded through the soft mud, 10 or 15 feet to the bedrock below. Surrounding
the pylons are several layers of water-resistant stone. As long as the sea
washes against this lower level, the structure above is well protected.

But now, the flooding is rising beyond the stone foundation to the building
itself, which, in most of Venice, is made of brick. The bricks, sometimes
coated in stucco, are soft and porous and much more vulnerable to corrosion.
Chemists at the University of Venice are studying how this works.

GUIDO BISCONTIN (University of Venice): It's very interesting to
see the effect of the water on the sample, which is no different from what
happens to the walls in Venice. I'm immersing the brick in a few
millimeters of water that contains a small quantity of salt.

NARRATOR: The salt travels up through the brick. As the water
dries, the salt crystallizes. With every new flood, the salt dissolves once
again and bores a little more into the brick. Eventually, the brick will
crack and crumble away. This is what's happening inside all the walls of
historic Venice.

Almost everywhere, ground floors are damp and moldy. Many residents have
moved upstairs or out of town. Since the 1950s, Venice has lost over half its
population. Today, fewer than 70,000 people live here.

Residents are trying to escape the flooding, but they're also running away
from the tourists. About 15 million tourists pour into tiny Venice every year,
and most of them stay less than a day.

They drive prices up on everything from food to rubber boots. So every day,
Venice becomes less of a living city, and more like a museum, a museum which is
often very, very wet.

But does it have to be this way?

GIANFRANCO VIANELLO (Restaurant owner): Just look at the
Netherlands, the whole country's below sea level and not a drop gets in.
I don't see why we, who are nearly three feet above sea level in St Mark's
Square, are not able to stop this water.

NARRATOR: The Netherlands used to be one of the most
waterlogged nations on earth, with more than half the country below sea
level. Since the 1600s, legions of windmills have powered water wheels
that lift water up into canals. These channels flowed back to the sea,
which was held at bay by earthen dikes.

But this system was not foolproof. In January 1953, hurricane-force winds
and an unusually high tide blasted the Dutch coast, collapsing the protective
banks. Eighteen hundred thirty five people were drowned and 70,000 were
left homeless. A third of the Netherlands was under water.

Over the next 50 years, the country spent billions walling itself off from
the North Sea. But for Rotterdam, the biggest port in the world, a permanent
barrier would have spelled economic disaster.

And so the Dutch came up with this: two mobile gates that can swing out to
block the river whenever the North Sea gets out of hand. To withstand the full
force of the sea, the structures must be gigantic. Each gate has a steel
barrier, seven stories tall, which holds back the water. The arms are as long
as the Eiffel Tower is high.

The gates are far from subtle, but Pier Vellinga, an environmental
scientist, is still a fan.

PIER VELLINGA (Vrije University, Amsterdam): When the North Sea
is rough, we are very vulnerable. And we had the choice between raising
all the dikes three, four, five meters, and rebuilding part of the old cities
or a one-time mobile barrier. And the population was much in favor of this
mobile barrier because it is safer and it has less effect overall on the
landscape.

NARRATOR: When the sea's calm, the barriers rest in canals on
the riverbank. If a major storm is predicted, the huge arms will rotate toward
the center of the river until they almost meet in the middle. Then the barriers
fill with water and sink into the riverbed.

Finished in 1997, the gates haven't been tested against a real storm yet,
but it's predicted they will be needed, on average, once every 10 years. The
Dutch system of dams and gates has been hailed as an engineering marvel, but
could it work for Venice?

PIER VELLINGA: What I know of Venice, the people and the visitors do not
like to see such a massive structure. And the design which is now
discussed for Venice is a more elegant solution.

NARRATOR: The Venetian flood plan calls for several sets of
mobile gates placed at the three large entrances to Venice's lagoon. Most of
the time, the steel gates lie flat in a special housing on the bottom of the
inlet. When extremely high tides threaten the city, compressed air is pumped
into the hollow gates, causing them to tilt upwards to the surface.

Each gate measures about 65 feet wide, about 12 feet thick, and 65 to 100
feet tall. They're designed to hold back a high tide more than six feet above
normal. When in use, they are supported by the water on both sides. After the
tide recedes, water flows into the gates, sinking them back into place.

If the hollow gates look flimsy, take a peek inside. Each gate is supported
by an internal steel framework weighing between 300 and 400 tons.

CHIANG C. MEI: The design of the mobile gate system is very
innovative. When it's in use, the elements of the barrier are allowed to
swing back and forth with the waves. In this way, much of the wave force
is transmitted back to the water on both sides, and very little force is
transmitted to the foundation and to the supporting structure. So in
this aspect, I think this is very, very clever.

NARRATOR: Many engineers believe these mobile gates represent
the salvation of Venice. There's just one problem: they don't exist,
except in computer simulations like this.

Since the flood of 1966, Italians have been talking about how to protect
Venice. Talking. And talking. But somehow, they've been unable to make a
decision. Instead, the gates have become a political hot potato, tossed from
one administration to the next. And that's meant a lot of tossing. Since 1966,
there have been more than 35 different governments in Italy.

ALBERT AMMERMAN: The politicians who are sitting in Rome, who
essentially have control of this, who are coming and going, almost on a yearly
or two year basis, do not have a clear focus on this. They will not be
in office when this is done, because it will take ten years. And this
helps to explain why you have this sort of inertia.

NARRATOR: It's not that nothing has been done. Every now and
then, there's a flurry of activity—like in the late 1980s, when engineers
rolled out a full-size prototype of a single gate, which was attached to a
large derrick. The whole contraption was ceremoniously hauled along the
coastline to gain public support.

WOMAN: On television they talked about this thing. It's the first time
I've seen it.

NARRATOR: After years of inaction, Venetians were
understandably skeptical.

SAILOR: Well, now, look. As far as efforts to save Venice go, up
to now they haven't tried very hard. They've gobbled up a lot of the
money meant for saving Venice, but they've done very little. We hope
that now things go well with these mobile gates.

VINICIO GROSSI (Gondolier): Okay, the politicians they
make project, blah, blah, blah, blah. Make all the Venetian
people now wait.

NARRATOR: Today, 14 years later, the Venetians are still
waiting. Construction of the gates has not even started. This is particularly
frustrating to engineers like Andrea Rinaldo.

ANDREA RINALDO (University of Padua): Especially in this country,
science and technology take a back seat to politics. It's never
intentional. It happens everywhere. But in a humanistic country
like this, what an MIT professor says and what a gondolier says is basically
the same thing. And if I say this, it's because I've been burned on this
before.

NARRATOR: Initially, the gate plan required years of testing.
No existing mobile gates operate in this way, popping out from the sea floor by
means of compressed air. To make sure the concept would work, engineers
spent four years fine-tuning the technical design of the single gate
module.

Then, to test the whole system, they set up several scale models of the
lagoon inlets complete with mini-mobile gates. Tiny ripples simulate the
effects of a mighty sea storm. And the response of the barriers is carefully
noted.

During the testing phase, engineers did find a potential problem. When test
waves hit the gates in certain patterns, adjoining gates rocked in opposite
directions, creating holes through which floodwaters could flow.

CHIANG C. MEI: This, I believe, was uncomfortable to the designers.
But there have been improvements of design by changing the dimension.
So this problem is solvable.

NARRATOR: By making a few minor changes in the size and angle
of the gates, the engineers are confident they'll avoid this problem, and the
gates will do their job. That is, if the gates ever get built.

Paolo Pirazzoli was born in Venice and has been fighting the gate plan for
years.

PAOLO PIRAZZOLI (National Center of Scientific Research): In
theory, the mobile gates, with optimal weather conditions, could put an end to
high tides. But only in theory, because it has a lot of problems.
It's an extremely complicated project. It's fragile and expensive in terms
of its construction and maintenance. And it would have a negative impact
on the environment.

NARRATOR: Environmentalists make up the biggest bloc against
the gates. They worry that the gates will be closed too often and will damage
the lagoon ecosystem. Some insist that Venice and the lagoon would be better
off left to Mother Nature. But the gate engineers don't buy that
argument.

ANDREA RINALDO: In recent years, there's been a growing sense among
people that every intervention in the lagoon and in the city is considered
"tampering." All these interventions are generally called "the disaster
of doing." It's hard for me to agree with this, because it's clear that
everything that we see today in the Venetian lagoon, it's the result of an
artificial system. It's not the product of natural evolution, but of a
massive interference from humans.

NARRATOR: It's true. Humans have been tampering with Venice
and its lagoon for hundreds of years. Today, Venice is a collection of built up
islands connected to the mainland by an artificial causeway. But sixteen
centuries ago, it was a very different place, a bunch of marshes surrounded by
a shallow mix of fresh and salt water.

The earliest settlers were probably running away from Attila the Hun and
other barbarians. They picked the spot precisely because it was so
inconvenient. Foreign invaders gave up trying to get there, figuring it wasn't
worth the trouble. Only the locals knew the location of channels deep enough
for seafaring vessels.

Safe from invasion, the ambitious Venetians developed a monopoly on trade
and shipping in the Eastern Mediterranean. This is what made Venice rich and
powerful. But by the 14th century, nature threatened to undo the
city's cozy place in the lagoon.

ENNIO CONCINA (University of Venice): The big turning point
happens in the middle of the 14th century. It seems that
reeds and swamps are invading the lagoon and menacing the city. The
swamps corrupt the water and bring disease and death. And in addition,
they make the city vulnerable to attack.

NARRATOR: Several rivers which ran into the lagoon were
depositing large amounts of silt. Only a foot or two deep, the lagoon was
becoming even shallower and more swamp-like. Mosquitoes and malaria were
rampant. Many feared the lagoon would fill in completely, and Venice would lose
its best system of defense. So the Venetians decided to take control of their
destiny.

ENNIO CONCINA: Essentially, there is a decision to proceed
systematically, to adjust everything that surrounds the lagoon, and to heal the
relationship between water and land through constant control.

NARRATOR: After much debate, the Venetians launched an
enormous public works project. Over the course of two centuries, they
built a series of large canals, diverting the major rivers around the lagoon so
the silt would be deposited elsewhere. The plan worked. The lagoon
stopped silting up.

But the Venetians were left with another problem, one they could not solve
so easily: their city was sinking. In fact, Venice had been sinking
since the very beginning.

The Alps are partly to blame. The weight of this mountain range is bearing
down on all of northern Italy, slowly driving it into the sea. But even more
damaging to Venice is what lies below.

CHARLES E. MCCLENNEN: Underneath Venice is a salt marsh. Venice
is sitting on several layers. The uppermost and thinnest are the lagoon
sediments, fairly muddy and, in part, salt marshes, and that's what Venice was
built on. Below that is nearly a mile thick of river sediments.
Those sediments are slowly compacting.

NARRATOR: In effect, Venice is sitting on a giant sponge
filled with water. As the weight of Venice pushes down on the sediments, it
squeezes some of the water out, and the sponge gets thinner.

To make matters worse, the seas have been slowly rising for centuries. This
is what the people of the lagoon have had to contend with since they first
arrived on these marsh islands, nearly 2,000 years ago.

Remnants of their long struggle can be found on the island of Torcello,
northeast of Venice. The land here has sunk so low, only a handful of residents
remain. Beneath the foundations of this 900-year-old cathedral, archeologist
Albert Ammerman has uncovered evidence of how the old lagoon dwellers dealt
with flooding.

ALBERT AMMERMAN: What we can always see in the archeological record is
the gradual, progressive buildup of the land surface. We can see five or
six floors, with just one after the other, six inches, a foot, gradually being
built up. Flooding would always be a problem, so their way to deal with
it was essentially to come in and continually be adjusting the ground level
upward, layer after layer after layer.

NARRATOR: Without the constant drone of water pumps, the
entire pit would be a giant swimming pool. But everything here was once high
and dry, safe above the tides.

ALBERT AMMERMAN: Well, the earliest evidence that we have is a walkway
made of Roman tiles, and that goes back to about 200 A.D.—we're in Roman
times—and this is found at five feet below sea level today.

NARRATOR: Anything built on these marshes would eventually
sink into the sea. So when a building was too-often flooded, the Venetians
would either raise up the floor or they might tear the whole thing down and
build a new structure on the old foundation.

But at some point in the past, the Venetians abandoned this
strategy.

ALBERT AMMERMAN: One of the fascinating things is that, since
essentially 1800, the time of Napoleon, the fall of the Venetian republic, they
have stopped doing that. Venice has become, in some sense, a museum.
It's become fossilized. The life of the city, also the notion of
preservation heritage, stops people from doing what was the thing that
Venetians always did, that is, build up the ground level.

NARRATOR: It's been a long time since the ground in Venice was
raised on a regular basis. When these homes were built, their front steps would
have stood well above the highest tide. Now, coated with green algae, they're
visible only at low tide.

Some believe that if Venetians want to fight off flooding, they should
follow the example of their ancestors.

PAOLO PIRAZZOLI: The ground level has been increased many times in the
past, with demolition and reconstruction. Today, of course, it's not possible
to demolish, but you can still make some increases in ground level that would
be compatible with architecture and landscape.

NARRATOR: Raising the sidewalks and walls of the canals,
perhaps by a foot or less, could go a long way to protect the city from
moderate high tides, the most common cause of flooding. When it's done well, as
here in one section of Venice, the change is hardly noticeable.

Some people say that this should be the priority for Venice, not the mobile
gates.

EDMUND PENNING-ROWSELL: That will solve an enormous proportion of the
flooding problems. It won't solve all the flooding problems, but it will
probably solve half or three-quarters of them. And you could do that
over the next 10 years. It will cost money, but it will solve the
problem very seriously in that immediate time scale.

NARRATOR: Plans are in the works to slightly raise the ground
in the lowest parts of Venice, including St. Mark's Square where the labyrinth
of elevated walkways is an all too familiar sight during the winter
months.

Most critics of the gates support this approach, and some have suggested an
additional strategy: narrowing the three inlets to the lagoon. The smaller the
opening, they argue, the less water can enter the lagoon, and the lower the
tide. Depending on how drastic the change, it could lower the tide by eight to
12 inches.

PAOLO PIRAZZOLI: With narrowing of the harbor's mouths, the acqua alta
would have to reach almost four and a half feet to cause flooding, and this
level is reached only once every three years. This means that we would have the
same frequency of acqua alta as we had a century ago.

NARRATOR: But narrowing the inlets would not be a completely
reliable defense against acqua alta. It could only work for tides that flow in
and out of the lagoon fairly quickly, on a cycle of just a few hours.

But there are some extremely high tides that linger, like the one in 1966.
The high tide flowed into the lagoon and stayed for a day. In a situation like
this, whether the inlet is wide or narrow, the floodwaters will eventually get
in.

PIER VELLINGA: We have studied that in detail, and we came to the
conclusion that it only works with low tides, but with exceptionally high tides
that last for, say, 24 hours, even making the mouth more narrow does not work
at all. It would be a worthless solution.

NARRATOR: Narrowing the inlets is considered a soft solution
for Venice, and has been endorsed by some environmentalists. But its effect
could be less than kind because it would reduce the amount of clean seawater
flowing into the lagoon with each tide.

Today, the lagoon is the largest saltwater marshland in the Mediterranean
and is cherished by Venetians as a place of natural beauty. It's known
throughout Europe as a bird-watching mecca.

Tens of thousands of birds stop here during their annual migrations. They
feed on the plentiful fish and shrimp that thrive in the shallow, salty lagoon
waters. If the flow of new seawater into the lagoon is reduced, these creatures
will be deprived of oxygen and nutrients. In addition, without the cleansing
action of the tides, one of the lagoon's problems may rapidly get much worse:
pollution.

ALBERT AMMERMAN: Well, the water in Venice has quite substantial levels
of pollution. There's raw sewage. I have a boat in Venice and I
take people around in the boat. You can actually see soap suds coming
out of people's washing machines. You can see worse things coming straight from
their houses.

NARRATOR: Venice has no municipal sewage treatment, so most
homes pipe their sewage into the canals. On top of this, there are the
factories at the nearby industrial complex of Port Marghera, which only
recently stopped dumping chemicals into the water. And there's also
agricultural pollution, fertilizers that flow down the Po Valley and into the
lagoon. All this pollution is swept out of the lagoon and into the sea, every
day, twice a day, by the tides.

If narrowing the inlets would be harmful, then what kind of damage would be
done by the mobile gates, which would close off the lagoon completely? How long
would the gates actually be closed when high seas threatened?

Recently, nature provided the engineers with another opportunity to test
their gates' performance, at least virtually. On November 6, 2000, tidal charts
in Venice called for a modest high tide, just a few inches above the mean.
Later that day, meteorologists forecast that a storm would push the tide two
and a half feet higher, which would cause some flooding. But the storm took a
sudden turn for the worse, and by 9 p.m., the tide had risen to four feet,
enough to flood 93 percent of the city.

Afterwards, researchers created a computer simulation of the storm to see
how effective the mobile gates might have been.

MARIA TERESA BROTTO (Consortium for a New Venice): The gates
would have been shut for nine hours, and the water level in the lagoon would
have reached not more than two feet, three inches.

NARRATOR: The engineers believe that once the gates were
opened again, tidal flushing would soon clean out any built up pollution. But
what about repeated closures over the course of an entire season? What effect
would these have on water quality?

According to Pier Vellinga, who was on a government panel which evaluated
the gates, not much.

PIER VELLINGA: Well, I'm a professor in environmental sciences myself,
and we have taken the concerns of the Green people very serious. But
when we studied it, we came to the conclusion that the closure of the barrier,
say 10, 20 times a year, has about the same effect as when you have lower tides
in summer. The claim that these mobile gates would really be bad for the
environment, we could not find any ground for.

NARRATOR: If the gates were in place today, they would
probably be closed, on average, about seven times a year. Even most
environmentalists agree this is not a problem. But what will happen in the
future?

Remember, Venice is sinking and the sea is rising. Won't the gates have to
be closed more often?

That depends on how much the city sinks and the sea rises in the next 100
years, the expected life of the mobile gates. Figuring that out is tougher than
it might seem, because in the middle of last century, Venice started to sink at
an unusually high speed.

The problem was soon traced to Port Marghera, where factories were pumping
large amounts of groundwater out from deep under the city.

CHARLES E. MCCLENNEN: The groundwater wells were active from the `30s
through the early 1970s, I believe, when they were stopped. And they were
stopped because it was realized that it was actually drawing the city down at a
faster rate than it would be sinking naturally.

NARRATOR: The pumping was a catastrophe for Venice. In less
than a century, the city lost about nine inches against the sea. But when the
pumping stopped, the sinking slowed down drastically. In the last couple of
decades, Venice sank very little.

No one knew how fast Venice would sink in the future, so the gate planners
chose several possible scenarios. One scenario was very conservative,
predicting that Venice would sink less than two inches in the next hundred
years. Before long, this choice was coming under attack, and from an unlikely
source.

ALBERT AMMERMAN: We found that the scenarios, the projections that they
were using in these impact studies were completely wrong. There were
fundamental flaws.

NARRATOR: Albert Ammerman is an archeologist not an engineer.
But together with marine geologist Charles McClennen, he had uncovered
controversial new evidence about the changing sea level of Venice.

CHARLES E. MCCLENNEN: There was no motivation on our part to study or
evaluate the mobile gates. Our initial purpose was really to look at the
archeology. Inadvertently, we learned about sea level in the lagoon of
Venice.

NARRATOR: By carefully measuring and dating the different
levels of ancient buildings, the team tried to determine how fast the seas in
Venice have been rising throughout her long history.

ALBERT AMMERMAN: The result of this was we could see the average rate of
change was five inches per century.

NARRATOR: The team believed the gate planners had been too
optimistic about Venice's rising sea level. And they began to question some of
the planners' other conclusions, particularly their predictions about global
warming. Global warming is a contentious issue, especially when it comes to its
effect on sea level.

Hollywood films have had a field day with the doomsday scenarios resulting
from melted polar ice caps. Waterworld, starring Kevin Costner, depicts
a future where dry earth is a hot commodity.

MAN IN WATERWORLD: Dirt!

NARRATOR: And Stephen Spielberg's A.I. transforms
Manhattan into a waterlogged ghost town. If New York is going to look like
this, then Venice is a goner, too.

But we don't have to melt all the ice on Earth to have problems. Even with
a little bit of global warming, the oceans will expand, that's what water does
when it's heated. And if the oceans get bigger, there's nowhere to go but
up.

But predicting how far up and how fast is extremely tricky.

PIER VELLINGA: We sometimes think that the oceans are like one tub of
water, and when it gets hotter, the water expands and rises. But if the
climate changes, then also the wind changes and the storm surges change.
So the water will be pushed up differently in different parts of the
world.

NARRATOR: Official estimates for global sea level rise in the
next hundred years are all over the map—from four inches to three feet.
Exactly what's going to happen in Venice is anybody's guess. The gate planners
estimated a probable rise of about eight inches, meaning that the barriers
would be closed just a few times a year.

CHIANG C. MEI:For this kind of sea level rise, it is probably
necessary to close the gate approximately 12 times a year or 45 hours per
year. The length of closure is very, very small compared to the time
when tidal flushing can be effective.

NARRATOR: But gate critics think that these figures are too
optimistic.

ALBERT AMMERMAN: If one accepts notions about global warming, what
reasonable scientists around the world think will happen is that over the next
100 years, the midpoint range of what they think will happen is that there will
be an 18 inch rise. If that happens, then you're going to have acqua
alta all the time and then you're going to have an emergency situation.

CHARLES E. MCCLENNEN: The problem that I see with the mobile gates in
the lagoon of Venice is that the closing may have to be too frequent and for
too long duration each time.

ALBERT AMMERMAN: What we're talking about is, in a bad year, out in the
later years, maybe after about 2060, 2070, you could have, day after day, week
after week, some days even two closings.

NARRATOR: Ammerman and McClennen fear that with so many
closings pollution inside the lagoon could build to dangerous levels.

But what will pollution levels be like sixty years from now? Aerial
surveys, along with sampling, have shown that water quality in the lagoon has
improved somewhat over the last decade. And it would be nice to think that at
some point in the next 60 years, Venice will build a sewage treatment
facility.

But if the worst-case scenarios for global warming come true, it won't
matter how clean or dirty the water is. There will simply be too much of it.
For some, this is reason enough not to build the gates.

EDMUND PENNING-ROWSELL: Well, in the next 100 years, Venice has
undoubtedly got a serious problem, because all the projections for sea level
rise indicate a rise of around 50 centimeters to 100 centimeters. And
that is going to mean that the mobile gate solution, which has been put
forward, will be impossible to implement.

NARRATOR: But when others consider the rising seas, their
conclusion about the gates is...

PIER VELLINGA: Well, exactly the opposite. Because we know so
little, and we think the lagoon is precious, and Venice is precious, the mobile
gates is like a precautionary measure. It will be very useful for at
least hundred years, and if sea level comes quicker than you think, at least
you have a way to save Venice.

NARRATOR: But are these mobile gates the best way to save
Venice? Ammerman can't help wishing there were more choices.

ALBERT AMMERMAN: What you're going at with the gates is one big final
thing. It's like trying to throw a long touchdown pass or hit a home
run. And what's going to happen if you miss, and that doesn't work?
You have nothing in place as a building block for the next generation, to
take you out to 2100 or 2200.

NARRATOR: He supports more studies and openness to new ideas.
The engineers say, with or without more research, it's time to build.

ANDREA RINALDO: As a general rule, I suspect that "wait and see" when
you deal with a phenomenon that change...on a global change scales,
would be rather unsuitable procedure, especially if you have to protect
treasures of a human kind like the city of Venice, its art, its heritage, its
architecture.

PIER VELLINGA: I think you first have to act and learn
simultaneously. If you only want to study and the sea level rises
quickly, you lose Venice and you lose the lagoon. You lose both.
So we think this solution is very good for the short term. And if you
postpone and study, and study, and study, we may study until the sea has risen
50 centimeters. Then you're not safe.

NARRATOR: Today, the Italian government seems intent on
building the gates, which could take eight to ten years. If the gates are
constructed, there is little doubt that they will stop the worst floods, at
least for a while.

But at some point, the mobile gates may not be enough to hold back the
rising sea. Having invested so much, will the government be able to launch an
even bigger project if the gates become obsolete?

EDMUND PENNING-ROWSELL: In the long term, that temporary solution,
costing perhaps three billion U.S. dollars will have to be repeated by
something more permanent. Venice and its lagoon will have to be sealed
off from the sea.

NARRATOR: What would it take to create a permanent solution
for Venice? There seem to be two main possibilities: wall off the city from the
lagoon, or wall off the lagoon from the Adriatic with permanent dikes,
transforming it into a fresh water lake. For a city so intimately linked to the
sea, both proposals seem unthinkable.

But the water is rising. Eventually, something will have to give.

CHARLES E. MCCLENNEN: The Venice you know today cannot be preserved as
it is today. Because all the data we have on global sea level and local
sea level is to our disadvantage when it comes to the city of Venice. It
was built on a salt marsh, at sea level, in a sinking area, and unfortunately
sea level is rising.

NARRATOR: For now, those who love Venice hold their breath,
hoping that the city that has defied time for so many centuries will somehow
manage to hold on.

Rising sea levels, shipping channels, the weight of the city itself—on
NOVA's Web site use a clickable map to see all the ways that Venice is under
siege, at PBS.org or America Online, keyword PBS.

To order this show or any other NOVA program for $19.95 plus shipping and
handling, call WGBH Video at 1-800-255-9424.

Next time on NOVA: He has a madness no medicine can cure. "I think about
plants more than anything else." And it drives him to the ends of the earth. "I
need to go to a place where no botanist has ever been...I've got it, I've got
it, I found it I found it." Orchid Hunter.

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